Facsimile machines and printers commonly use thermal recording devices that record data by means of thermosensitive paper and transfer type thermosensitive recording media. In such a recording apparatus, a thermal recording head with heating elements arranged in a row is normally used. The thermal head produces thermal energy during recording and the image quality may deteriorate because of the cumulative heat. One of the primary factors causing image deterioration is the fact that the resistances of the heating elements differ from one another.
The variations in resistance are mainly attributed to the process of manufacturing the heating elements and may be roughly classified into two categories. The first cause is that the heating elements in the thermal head vary in resistance, and the other cause is that mean resistance of the heating elements in one thermal head differs from that in a different head. Moreover, the variations in resistance may be large in some cases; e.g., about .+-.25% in the first case and within the range of 200-300 ohms (AC) in the second case.
In a recording head whose heating elements have resistances that are different from one another, the printing density will be uneven even if the same amount of energy is applied to each element. The variation in printing density makes desired gradations in printed images difficult to reproduce with precision. This problem is even more serious in a color data recording apparatus required to record with several colors. If the heating elements have resistances that are different from one another, tone reproducibility is erratic and the quality of a recording deteriorates seriously.